1. Field of the Invention
The present invention relates to an image forming apparatus using colored toner and transparent toner. In particular, the present invention relates to an image forming apparatus capable of concurrently reducing an occurrence of “scattering” and “re-transfer” when a toner image on an image bearing member is transferred onto a recording material to be borne on an intermediate transfer member or a recording material bearing member.
2. Description of the Related Art
Conventionally, as an image forming apparatus using an electrophotographic process, a copying machine, a printer (e.g., a laser beam printer, an LED printer, etc.), a facsimile machine, a word processor, and the like have been widely used. In the image forming apparatus of the electrophotographic process, an electrostatic image (latent image) is formed on an image bearing member that is an electrophotographic photosensitive member (photosensitive drum) generally having a cylindrical shape, the electrostatic image is developed with toner to form a toner image, and the toner image is finally transferred onto a recording material and fixed thereon, whereby an image is output.
Recently, even in the image forming apparatus of the electrophotographic process, there is an increasing demand for an apparatus capable of forming a color image on the recording material by superimposing a plurality of kinds of toner images.
Herein, conventionally, the color image forming apparatus of the electrophotographic process, for forming the color image on the recording material by superimposing the plurality of kinds of toner images, employs the following systems. There is a system (direct transfer system) in which first toner images successively formed on the image bearing member are transferred onto the recording material borne on the recording material bearing member serving as an image transfer member in a transfer portion every time the toner image is formed, and the plurality of kinds of toner images are superimposed on the recording material. Further, there is a system (intermediate transfer system) in which the toner images successively formed on the image bearing member are transferred onto the intermediate transfer member serving as the image transport member in a primary transfer portion every time the toner image is formed, the plurality of kinds of toner images are superimposed on the intermediate transfer member, and then, a multi-layered toner image is secondarily transferred onto the recording material at once. Still further, as the image forming apparatus using the plurality of kinds of toners, so-called tandem-type and rotary-type are widely known.
In the tandem-type image forming apparatus, developing devices filled with developers having different spectral characteristics are provided so as to correspond to the plurality of image bearing members. Image forming portions including the image bearing members and the developers, respectively, are placed in series in a movement direction of the image transport member (recording material bearing member, intermediate transfer member).
On the other hand, in the rotary-type image forming apparatus, a plurality of developing devices are provided so as to correspond to one image bearing member. The plurality of developing devices are provided to a rotatable rotary portion serving as developing device holding means. Then, by rotating the rotary portion, development is performed while the developing devices used are switched. For example, in the case of adopting an intermediate transfer system, the toner image is primarily transferred onto the intermediate transfer member every time the toner image is formed on the image bearing member by each developing device, whereby the multi-layered toner image is formed on the intermediate transfer member. After all the kinds of toner images have been transferred, the toner image on the intermediate transfer member is secondarily transferred onto the recording material.
As a method of transferring the toner image from the image bearing member to a member on which the image is transferred, there has been widely adopted a method of transferring toner from the image bearing member to the member on which the image is transferred electrostatically by applying a bias having a polarity opposite to a charging polarity of the toner from a back surface of the member on which the image is transferred in the transfer portion between the image bearing member and the member on which the image is transferred.
However, in the above-mentioned image forming apparatus in which the toner images are superimposed, when the toner image is transferred onto a recording material or an intermediate transfer member, there arises a problem in that “toner scattering” and “re-transfer” occur, thereby making it impossible to obtain an image of high quality. In this case, the “toner scattering” and the “re-transfer” have an antinomic relationship, so it is difficult to suppress both of them concurrently.
Hereinafter, mechanisms of the occurrence of “toner scattering” and “re-transfer” will be described.
First, the occurrence mechanism of toner scattering phenomenon will be described.
FIG. 13 schematically shows the vicinity of a primary transfer portion N1 of an image forming apparatus adopting the intermediate transfer system. In the primary transfer portion N1 shown in FIG. 13, a primary transfer roller 5 serving as the primary transfer means is opposed to a photosensitive drum 1 serving as the image bearing member via an intermediate transferring belt 21 serving as the intermediate transfer member. The primary transfer roller 5 presses the intermediate transferring belt 21 toward the photosensitive drum 1, whereby the intermediate transferring belt 21 is brought into contact with the photosensitive drum 1. An arrow E of FIG. 13 represents a transport direction (surface movement direction) of the intermediate transferring belt 21. In the primary transfer portion N1 shown in FIG. 13, areas A, B, and C have the following meanings.
The area A (contact area) represents a width of a contact nip (first contact nip) n1 formed between the photosensitive drum 1 and the intermediate transferring belt 21, along the movement direction of the intermediate transferring belt 21.
The area B represents a width of a contact nip (second contact nip) n2 formed between the intermediate transferring belt 21 and the primary transfer roller 5, along the movement direction of the intermediate transferring belt 21.
The area C represents a width of a portion (hereinafter, referred to as “upstream-side gap nip”) that extends off an upstream side along the movement direction of the intermediate transferring belt 21 relatively with respect to the first contact nip n1 in the second contact nip n2.
In the following description, the surface (first surface) with respect to an image transport member (intermediate transfer member, recording material bearing member) refers to a surface on a side in contact with the image bearing member, and the back surface (second surface) refers to a surface on an opposite side thereof.
The transfer of the toner image formed on the photosensitive drum 1 to the intermediate transferring belt 21 is usually performed in the area A where the first contact nip n1 and the second contact nip n2 are overlapped on the intermediate transferring belt 21.
However, in the case where the upstream side gap nip area C is present, transfer also occurs in the gap nip area C. That is, in the upstream side gap nip area C, the photosensitive drum 1 and the intermediate transferring belt 21 are opposed to each other with a gap interposed therebetween without forming a contact nip. Therefore, there occurs so-called “pre-transfer” in which a toner image on the photosensitive drum 1 scatters in an air layer to be transferred onto the intermediate transferring belt 21. Pre-transfer causes the toner scattering in an end of a toner image, which degrades image quality.
In order to avoid pre-transfer, it is effective to offset a center (i.e., the center of the area B) B0 in the movement direction of the intermediate transferring belt 21 of the second contact nip n2 formed between the intermediate transferring belt 21 and the primary transfer roller 5, to a downstream side in the movement direction of the intermediate transferring belt 21 relatively with respect to a center (i.e., the center of the area A) A0 in the movement direction of the intermediate transferring belt 21 of the first contact nip n1 formed between the photosensitive drum 1 and the intermediate transferring belt 21. That is, in order to avoid pre-transfer, it is effective to increase a distance L1 of the primary transfer roller 5 from an upstream end A1 of the area A in the rotation direction of the photosensitive drum 1.
Assuming that a vertical line from the rotation shaft of the photosensitive drum 1 to the surface of the intermediate transferring belt 21 is a photosensitive drum center line X, the photosensitive drum center line X typically substantially matches with the center A0 of the area A. Further, assuming that a vertical line from the rotation shaft of the primary transfer roller 5 to the surface of the intermediate transferring belt 21 is a transfer roller center line Y, the transfer roller center line Y substantially matches with the center B0 of the area B. Thus, in this case, as shown in FIG. 14, the rotation shaft of the primary transfer roller 5 is placed so as to be shifted to a downstream side in the movement direction (direction represented by the arrow E) of the intermediate transferring belt 21, with respect to the rotation shaft of the photosensitive drum 1 by a distance L (mm) between the photosensitive drum center line X and the transfer roller center line Y.
With such a configuration, the upstream side gap nip area C can be reduced as much as possible. This can suppress the pre-transfer, thereby preventing a phenomenon of toner scattering.
Next, the occurrence mechanism of the re-transfer will be described.
The following was found: when the rotation shaft of the primary transfer roller 5 is placed so as to be offset to the downstream side in the movement direction of the intermediate transferring belt 21 with respect to the rotation shaft of the photosensitive drum 1 in order to prevent pre-transfer, a phenomenon (hereinafter, referred to as “re-transfer”) becomes more conspicuous in which a toner image having been transferred onto the intermediate transferring belt 21 by the previous primary transfer operations is reversely transferred from the intermediate transferring belt 21 to the photosensitive drum 1 during a subsequent primary transfer operation.
That is, in the rotation direction of the photosensitive drum 1, by increasing the distance from the upstream end A1 of the area A to the primary transfer roller 5, “re-transfer” becomes more conspicuous.
In FIG. 15, in a case where a yellow toner image is transferred onto the intermediate transferring belt 21 by the first primary transfer operation, and a magenta toner image is transferred onto the intermediate transferring belt 21 by the second primary transfer operation, the above-mentioned distance L in the primary transfer portion of the magenta toner image is varied. FIG. 15 shows results of a relationship between the re-transfer amount of the toner transferred onto the intermediate transferring belt 21 in the primary transfer portion of the yellow toner image and the transfer current.
An amount of toner (re-transfer toner amount) to be re-transferred is measured as follows. That is, a predetermined image is formed on the intermediate transferring belt 21 by the first primary transfer operation. Then, while the second primary transfer operation is being performed, the power supply of the main body of the image forming apparatus is turned off, whereby toner transferred onto the surface of the photosensitive drum 1 at this time is transferred onto an adhesive tape. A density of the toner obtained by allowing the adhesive tape to adhere to a white recording sheet and subtracting a reflecting density of a sample tape from the reflectance of a reference tape is determined to be a re-transfer toner amount.
As is understood from FIG. 15, when the rotation shaft of the primary transfer roller 5 is offset to the downstream side of the intermediate transferring belt 21 with respect to the rotation shaft of the photosensitive drum 1 so as to eliminate the upstream side gap nip area C, in order to prevent the pre-transfer, the amount of re-transfer toner increases.
Re-transfer is considered to occur as follows: among the toner transferred onto the intermediate transferring belt 21, toner having a small charged amount and toner reversely charged due to an influence of a discharge phenomenon in a transfer portion or a transfer current are transferred onto the photosensitive drum 1 side. When re-transfer occurs, the density of a final image may become low depending upon the degree of re-transfer.
As described above, suppressing pre-transfer to prevent a toner scattering phenomenon and reducing the amount of re-transfer toner have an antinomic relationship, and it is difficult to reduce the occurrence of “toner scattering” and “re-transfer” concurrently.